In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the physical properties of the media. Because the spread of the ink droplets striking the media is a function of the media surface properties and porosity, the print quality is inconsistent. To address this issue, indirect printers have been developed that eject ink onto a blanket mounted to a drum or endless belt. The ink is dried on the blanket and then transferred to media. Such a printer avoids the changes in image quality, drop spread, and media properties that occur in response to media contact with the water or solvents in aqueous ink. Indirect printers also reduce the effect of variations in other media properties that arise from the use of widely disparate types of paper and films used to hold the final ink images.
In aqueous ink indirect printing, an aqueous ink is jetted onto an intermediate imaging surface, typically called a blanket, and the ink is partially dried on the blanket prior to transfixing the image to a media substrate, such as a sheet of paper. To ensure excellent print quality the ink drops jetted onto the blanket must spread well and not poorly coalesce prior to drying. Otherwise, the ink images appear grainy and have deletions. The lack of spreading can also cause missing or failed inkjets in the printheads to produce streaks in the ink image. Spreading of aqueous ink is facilitated by materials having a high energy surface. In order to facilitate transfer of the ink image from the blanket to the media substrate, however, a blanket having a surface with a relatively low surface energy is preferred. These diametrically opposed and competing properties for a blanket surface make selections of materials for blankets difficult. Reducing ink drop surface tension helps, but the spread is still generally inadequate for appropriate image quality. Offline oxygen plasma treatments of blanket materials that increase the surface energy of the blanket have been tried and shown to be effective. The benefit of such offline treatment may be short lived due to surface contamination, wear, and aging over time.
One challenge confronting indirect aqueous inkjet printing processes relates to the spread of ink drops during the printing process. Indirect image receiving members are formed from low surface energy materials that promote the transfer of ink from the surface of the indirect image receiving member to the print medium that receives the final printed image. Low surface energy materials, however, also tend to promote the “beading” of individual ink drops on the image receiving surface. Since a printer partially dries the aqueous ink drops prior to transferring the ink drops to the print medium, the aqueous ink does not have an opportunity to be forced to be spread during the transferring/printing process. The resulting printed image may appear to be grainy and solid lines or solid printed regions are reproduced as a series of dots instead of continuous features in the final printed image. Consequently, improvements to indirect inkjet printers that improve the spreading characteristics of aqueous ink drops during an indirect printing process would be beneficial.